The title mol­ecule, C39H28N2O, is a well known dendron used in the synthesis of phenyl­azomethine dendrimers. The central benzophenone core is twisted, as expected, due to hindrance between H atoms: the dihedral angle between core benzene rings is 54.49 (5)°, identical to that of the stable polymorph of benzophenone (56°). For the same reason, phenyl groups substituting imine C atoms make a large dihedral angle, although similar for each imine: 71.83 (6) and 67.64 (5)°. The six aromatic rings in the mol­ecule thus seem to be quite randomly oriented, and such an arrangement is not favorable for efficient stacking inter­actions in the crystal. The same behaviour is observed in the vast majority of diphenyl­imino-containing organics. The low triclinic crystal symmetry may be a consequence of these features.

The title benzophenone derivative has been widely employed as a dendron in the synthesis of phenylazomethine dendrimers (DPAs), mostly in the group of Yamamoto at the Keio University (Higuchi et al., 2001; Takanashi et al., 2004; Yamamoto & Higuchi, 2004). This group and others reported on the preparation of a vast array of supramolecular entities with interesting properties. We became interested in preparing this dendron by using microwave heating, given that it is becoming an important method in laboratories worldwide: it is an environment-friendly technique for the efficient syntheses of organic molecules. The main advantages of microwave-assisted organic synthesis are shorter reaction times, minimum waste and generally higher yields, operational simplicity as well as reduction of thermal degradative byproducts along with cleaner work-up. As expected, better yields were obtained and we realized that, surprisingly, the crystal structure had not been reported so far.

The molecule (Fig. 1) crystallizes in the low symmetry space groupP1. The imine bond lengths, 1.2813 (18) and 1.2784 (19) Å, are as expected, however, N atoms significantly deviate from trigonality. Large C═N—C angles are observed, 127.61 (12) and 123.09 (13)°, probably because of the steric repulsion between the central benzophenone benzene rings and the diphenylmethylene groups. The central benzophenone displays a twisted conformation, the dihedral angle between benzene rings being 54.49 (5)°. This value is indeed close to that reported for benzophenone, 56° (orthorhombic phase, Fleischer et al., 1968) or 64° (metastable monoclinic phase, Kutzke et al., 2000). This conformation avoids any intramolecular H···H contacts. In the same way, diphenyl groups bonded to imine C atoms are twisted, by 71.83 (6)° (diphenyl group at C9) and 67.64 (5)° (diphenyl group at C29). These angles are common for diphenylimino-containing organics (range of angles retrieved from the CSD : 57 to 90°; CSD, version 5.31 with all updates; Bruno et al., 2002).

As a whole, the six rings in the molecule seem to be randomly oriented. This chaotic arrangement is consistent with the low crystal symmetry, and does not favor π···π or C—H···π interactions in the crystal structure. For example, the shortest intermolecular separation between centroids of two rings is 4.45 Å. The calculated packing index is indeed low for this polyphenyl molecule, 0.672. A search in the CSD for organic molecules containing the Ph2C═N fragment shows that more densely packed crystals in this class of compounds are scarce. For 151 hits, only two structures present symmetry-related diphenylimino groups with phenyl rings separated by less than 3.9 Å (Appel et al., 1985; Buhmann et al., 1993).

A modified procedure for improved synthesis of the title compound was used. The Higuchi's route (Higuchi et al., 2001; see compound 'dendron G2' in this paper) consists of the condensation between benzophenone and 4,4'-diaminobenzophenone in presence of DABCO (1,4-diazabicyclo[2.2.2]octane) and TiCl4, in chlorobenzene. In the original synthesis, the mixture was heated at 398 K for 24 h to afford dendron G2 in 48% yield. In place of thermal activation, we performed a microwave-assisted synthesis in a monomode MIC-1 oven (Tekno-lab, S.A.) with maximum power output of 600 W. Irradiation was applied for 20 min., affording the title compound with an enhanced yield of 65% after silica gel column chromatography (ethyl acetate:hexane = 1:5). Single crystals were obtained by slow evaporation of the eluate at 298 K.

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